Field of the Invention
The present invention relates to an insert for forming a high load carrying end connection in a uni-axial composite material. The present invention also relates to a method of forming an end connection in a uni-axial composite material using the insert. The present invention is exemplified by an insert for a root end of a wind turbine blade. However, the insert is also suitable for use in other applications and with other materials.
Description of the Related Art
In large horizontal axis wind turbines the wind turbine blades are connected to the rotor hub by a number of bolt attachment points. The hub end of the wind turbine blades (the root) is cylindrical in section and typically has a diameter of 1500 mm to 3000 mm. Approximately 60 to 80 bolts connect the blade to a radial pitch bearing within the rotor hub. The bolts are typically M30 to M40 size and each is required to withstand a pullout force of the order of 200 to 400 kN. The bolts are arranged circumferentially around the root.
The female part of the connection must be located in the root part of the blade so that the male bolts can be accessed for maintenance from the hub side when in service to ensure that there is no failure in the connection.
The root of the blade is typically manufactured from fibre reinforced plastic, typically glass fibre in epoxy, vinylester or polyester resin. The predominant fibre orientation in the root structure is uni-axial with the fibres running parallel to the axis of the blade/root cylinder, with very few fibres in the radial direction. The uni-axial orientation of fibres in the root structure presents a problem for the formation of the female part of the hub connection. This is because, if a female thread is cut directly into the “end grain” type uni-axial laminate of the root, the fibres will be cut resulting in a low pull out strength equal only to the shear strength of the resin. This is illustrated in FIG. 1 which shows that the uni-axial threads have been cut in the vicinity of the female thread such that only the resin provides structural integrity to the thread formation.
To overcome this problem long metallic female threaded inserts have been used. The metallic inserts are designed provide a large bond area so that, when the inserts are bonded into the uni-axial composite structure of the root, a bond having a sufficient pull out strength is achieved. The male bolts thread into the female threaded inserts to form the connection.
The metallic inserts are either added after the composite root structure has been cured, or when the root structure is being laminated/infused. In the first method, holes for the inserts are drilled into the root and the inserts are then bonded into position. This method requires specialist adhesive and equipment. In the alternative method, the inserts are placed into the uncured laminate during “lay-up” and are then cured into the structure when the root composite is cured.
Although the use of metallic inserts solves the problem of cutting female threads directly into the uni-axial “end grain” of the root structure, they have their own problems. For example, structural problems can be caused by the thermal mismatch between the metallic inserts and the surrounding composite material, which have different thermal expansion coefficients. In addition, the metallic inserts have a higher stiffness than the surrounding composite material leading to problems with flexural mismatch in service.
One way of countering these problems is to provide the metallic inserts with a tapered, more flexible, (sometimes referred to as carrot shaped) configuration to minimise the effect of material stiffness mismatch. In addition, the composite laminate in the region of the connection is made very thick, and hence stiffer, to further reduce the effect of the material stiffness mismatch. A typical schematic example of a prior art metallic insert 1 embedded in a uni-axial composite material 2 is shown in FIG. 2. In practice approximately one third of the length of the insert is tapered. The taper of the inserts used in the current art can also be made by tapering down the amount of metallic material on the inside of the insert.
As wind turbines have got larger, the extra composite material required at the root end to compensate for the structural mismatch between the metallic inserts and the composite laminate has become very significant. The additional material contributes greatly to the overall mass, and hence cost, of the blade. For a 40 m wind blade, the wall thickness of the composite laminate at the root end is in the order of 80 mm and, for a 350 mm metallic insert, the wall thickness must be maintained for approximately 500 mm before it can begin to reduce. Because of the need for large amounts of composite material in the root structure, the cost of using more expensive materials, such as carbon fibre composite as required for larger blades, becomes prohibitive.